Controller

ABSTRACT

A controller is disclosed for controlling the optical output of at least one light emitting diode. The controller comprises a control unit and a power supply unit for supplying power to the at least one light emitting diode. The control unit is arranged to receive as input, first and second signals which are representative of the operating characteristics of the at least one light emitting diode, and which is further arranged to control the power output from the power supply unit to the at least one light emitting diode in dependence of the first and second signals. The first signal is representative of the current within the at least one light emitting diode and the second signal is representative of the temperature of the at least one light emitting diode so that the optical output from the at least one LED can be varied in accordance with the operating characteristics of the at least one light emitting diode.

FIELD OF THE DISCLOSED TECHNOLOGY

The present invention relates to a controller and particularly, but notexclusively to a controller for controlling the optical output of atleast one light emitting diode.

BACKGROUND

The optical output from an LED will vary over its useful lifetime and assuch the perception of the light output will also vary.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSED TECHNOLOGY

We have now devised a controller for controlling the optical output ofat least one light emitting diode.

In accordance with the present invention as seen from a first aspect,there is provided a controller for controlling the optical output of atleast one light emitting diode,

the controller comprising a control unit and a power supply unit forsupplying power to the at least one light emitting diode,

the control unit being arranged to receive as input, first and secondsignals which are representative of the operating characteristics of theat least one light emitting diode,

and which is further arranged to control the power output from the powersupply unit to the at least one light emitting diode in dependence ofthe first and second signals, wherein

the first signal is representative of the current within the at leastone light emitting diode and the second signal is representative of thetemperature of the at least one light emitting diode.

The provision of two feedback loops enables the control unit to monitorand adjust the optical output of the light emitting diode and to providefor an energy saving lighting system, without compromising the visualperception of the optical output.

Preferably, the control unit further receives as input a third signalwhich is representative of the time and a fourth signal which isrepresentative of ambient lighting conditions.

The third signal is preferably generated by a clock which is arranged tomonitor the time, such as the time of day and/or month and/or year, forexample. The fourth signal is preferably generated by a sensor whichsenses ambient lighting conditions.

The controller is preferably arranged to control the optical output of aplurality of light emitting diodes.

The controller is preferably powered by an alternating current mainssupply. The power unit preferably receives as input a rectified directcurrent supply which is obtained from the alternating current mainssupply.

The power supply to the controller and the power unit is preferablyfirst regulated to minimise any voltage spikes.

In accordance with the present invention as seen from a second aspect,there is provided an energy saver light emitting diode (LED) powersupply system composed and realized as shown in the description with theelements represented in the drawing with the exposed automatic controlsand features, comprising:

an overvoltage suppression section to cut the voltage transient peaks ofthe AC grid; a Graetz Schottky diode bridge for the efficient highvoltage AC\DC conversion; a ripple filter to obtain a stable DC voltage;an high efficiency resonant DC/DC buck converter for an efficient DC\ DCconversion; a power unit that regulates efficiently voltage and currenton the LED module; a control unit to manage automatically the othersections and to performs protection and energy saving operations; a LEDmodule to convert efficiently the electrical energy supplied by thepower unit into light energy.

In accordance with the present invention as seen from a third aspect,there is provided a method of controlling the optical output of at leastone light emitting diode, the method comprising the steps of:

providing electrical power to the at least one light emitting diode tocause the light emitting diode to produce an optical output;

generating a first signal which is representative of the current passingthrough the at least one light emitting diode;

generating a second signal which is representative of the temperature ofthe at least one light emitting diode;

adjusting the electrical power supply to the at least one light emittingdiode in dependence of the first and second signals.

In accordance with the present invention as seen from a fourth aspect,there is provided a lighting system comprising an array of lightemitting diodes and at least one controller of the first or secondaspect.

An embodiment of the present invention will now be described by way ofexample only and with reference to the accompanying drawing, whichillustrates an electronic circuit comprising a controller according tothe described embodiment of the present invention.

Referring to the drawing, there is illustrated an electronic circuit 10comprising a controller 20 for controlling the optical output of anarray of light emitting diodes (LED) (not shown) housed within an LEDmodule 30. The LED's are powered using a power unit 21 which receives asinput a control signal from a control unit 22 and a power supply from apower modulation system 40.

The power modulation system 40 comprises a Graetz diode bridge 41 whichreceives an alternating current input from the mains 50, for example.The ac supply to the diode bridge 41 however, is first passed through avoltage suppression circuit 42 which is arranged to remove any voltagespikes which appear from the mains 42 above a threshold value, and afuse 43, such as a self restoring fuse, which is arranged to isolate thepower modulation system 40 from the mains 50 in the event that the mainsinput voltage far exceeds the average voltage.

The diode bridge 41 comprises four Schottky diodes 41 a-d which arearranged to minimize the voltage drop across the diode bridge 41, andthus minimize electrical power dissipation in the diode bridge 41.

The diode bridge 41 is arranged to generate a rectified voltage which issubsequently passed through a ripple filter 44 comprising a capacitorand resistor (not shown) arranged in a parallel configuration. Thecapacitors (not shown) of the filter 44 comprise a low equivalent seriesresistance (ESR) to withstand any peaks in the current, and comprise aworking temperature range which ensures a long filter lifetime. Theripple filter 44 is arranged to smooth the oscillating waveform from thediode bridge 41 and generate a substantially constant voltage, which issubsequently passed to a buck converter 45.

The buck converter is arranged to step-down the direct current voltagesupply from the filter 44 to a useful voltage for powering the powerunit 21 of the controller 20. The buck converter 45 regulates the outputvoltage to the power unit 21 to produce a stable, constant voltage, andincorporates over-current protection, short-circuit protection andover-voltage protection circuitry to protect the power unit 21 fromspurious voltages from the mains supply 50.

The power unit 21 is arranged to power the LED's of the LED module 30.The power unit 21 is arranged to generate a low frequency pulse widthmodulation of the voltage output from the resonant converter 45, tomodulate the energy to transfer to the LED module 30. In order tominimise winding power losses associated with the converter 45, theconverter 45 operates without the mean value inductor-capacitor (L-C)filter (not shown), which is commonly used in buck converters, withoutaffecting LED lifetime. This is because the LED's (not shown) of themodule 30 are capable of withstanding the peak forward current surges.

The mean value of the voltage across the LED module 30 can be expressedas:

$V_{M} = {\frac{1}{T}{\int_{0}^{T}{{V(t)}\ {t}}}}$

where V(t) is the impulse signal with variable duty cycle and with peakvalue equal to the output voltage of the buck converter 45.

The voltage output from the power unit 21 is controlled by a switch (notshown), such as a metal oxide semiconductor field effect transistor(MOSFET), which comprises a low drain-source resistance when arranged inthe ON state. The switching frequency of the MOSFET (not shown) must bea maximum of 3-4 kHz to ensure a high efficiency and to reduce anyvoltage stresses on the LED's (not shown) of the module 30.

The power modulation system 40 is also arranged to power the controlunit 22. The output from the fuse 43 and the voltage suppression circuit42 is passed through a current transformer 46 and then rectified andfiltered using circuit 47 to produce a substantially stable uniformvoltage. The output from the circuit 47 is subsequently passed to avoltage regulator 48 which further stabilizes the voltage supply to thecontrol unit 22, such that the control unit 22 can maintain control ofthe entire electronic circuit 10 and perform operations such asmodifying the power supply to the buck converter 45 and disabling apower factor correction of the buck converter 45 to increase conversionefficiency at low output current.

The control unit 22 comprises a microprocessor (not shown), or any otherkind of programmable integrated circuit such as a field programmablegate array (FPGA), which is capable of controlling the power unit 21 ingenerating a pulse width modulated (PWM) signal. In this way the amountof power transferred to the LED module 30 is proportional to the dutycycle (D) of the PWM signal generated by the power unit 21. Uponincreasing the duty cycle, more power will be delivered to the LED's(not shown) within the module 30 per voltage period.

When the mean value of the output current is equal to the nominal valueof LED module 30 operating output current, the condition D<1, must besatisfied. This is because the human eye is more sensitive to the peakof light intensity than the mean value, while power consumption isproportional to the mean value of the current absorbed. In this way, itis possible to obtain a better visual perception using less electricalpower.

The control unit 22 is arranged to set the duty cycle value following acontrol algorithm which has input values relating to the current withinthe LED's (not shown) of the module 30 as determined by a current sensor23, the temperature of the LED's (not shown) as determined by atemperature sensor 24, ambient light intensity as determined by a lightsensor 25, and a time signal, such as the time of day and/or time ofyear, as generated by a clock 26.

The signal from the current sensor 23 is necessary for regulating thecurrent flow within the LED's (not shown) and for short-circuitprotection of the power unit 21. The current signal from the LED module30, is further passed through a low pass filter 27 to generate anaverage current signal which is passed to the control unit 22.Accordingly, when the mean value of the current flowing in the LEDmodule 30 deviates outside a pre-defined range, the control unit 22 isarranged to adjust the duty cycle applied to the power unit 21 to varythe “ON” time during the voltage period and thus vary the current whichis output therefrom.

When the mean value of the current flowing in the LED module 30 exceedsa maximum permitted current value for example, the control unit 22 isarranged to generate a signal which causes the power unit 21 to switchto the OFF state. The current sensor 23 is of an electromagneticinductive type to reduce losses associated with sensors comprisingamperometric resistive shunts, thereby increasing the efficiency atwhich electrical power is converted to optical output from the LED's(not shown). The current sensor 23 may comprise a transducer (not shown)comprising a wire winding formed on a ferromagnetic ring (not shown),which must be crossed by one of the two wires (not shown) of the LEDmodule 30.

The temperature sensor 24 which monitors the temperature of the LED's(not shown), is mounted on a circuit board (not shown) of the LED module30 and is arranged to generate a signal to the control unit 22 which isrepresentative of the temperature of the circuit board (not shown), andthus the LED's (not shown). When the temperature of the LED module 30deviates outside a pre-defined range, for example above 60° C., thecontrol unit 22 decreases the duty cycle of the signal which is appliedto the power unit 21 in proportion to the deviation in the temperatureof the LED's outside the pre-defined range, to vary the mean value ofthe current which is output from the power unit 21, and thus thetemperature of the LED module 30. This kind of temperature control isrequired to extend LED lifetime and to make the LED's (not shown)operate near the point of maximum device efficiency. This is because LEDluminous efficiency is inversely proportional to the temperature of theLED, and so by keeping this temperature as low as possible, the LEDefficiency can be held at its maximum value.

When the temperature of the LED module 30 exceeds the maximum operatingtemperature of the LED employed, the control unit 22 is arranged togenerate a signal causing the power unit 21 to switch OFF the powersupply to the module 30, and thus protect the LED's (not shown).

The light sensor signal is required when, for example, there is a needto lower the output light intensity of the LED module 30, thereby savingan additional amount of energy. The output from the light sensor 25 maybe sensitive to an external command, such as a remote control signal ora signal from a resistive potentiometer or a trimmer (not shown), forexample. The signal from the light sensor 25 may be a binary serialsignal or an analogue signal which is sampled by the control unit 22.The signal from the light sensor 25 comprises information relating tothe variation in duty cycle of the PWM signal which is required torealise the reduction in electrical power supply to the module 30.

The real time clock 26 is arranged to monitor the current time and isarranged to deliver time and day information to the control unit 22, soas to affect the light output in accordance with the time of day forexample. The clock 26 is powered by a battery (not shown) and generatesa serial signal with an interface protocol which is recognized by thecontrol unit 22, such that the control unit 22 can provide for a gradualor stepped change in output light intensity.

The LED's (not shown) within the LED module 30 are arranged in anelectrical parallel arrangement of rows of LED's (not shown), with theLED's (not shown) of each row being arranged in an electrical seriesconfiguration. The number of LED's (not shown) in series in each rowmust be more than ten, to reduce power losses due to electrical Ohmicconduction and to improve the efficiency of the buck converter 45. Thisis because the converter 45 provides an increased efficiency when theoutput voltage is higher and the output current is lower.

The LED module 30 comprises a further row comprising a seriesarrangement of a resistor (not shown) and a Zener diode (not shown). Therow comprising the resistor (not shown) and the Zener diode (not shown)is arranged in parallel to the rows of LED's (not shown) and must beconnected such that the cathode of the Zener diode (not shown) iscoupled to the anode of the LED (not shown). This scheme is helpful as apassive over-voltage LED protection circuit, but is essential, when oneor more LED's of a series is interrupted or defectively soldered, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high level block diagram of a device of an embodiment ofthe disclosed technology.

1. A controller for controlling the optical output of at least one lightemitting diode, the controller comprising a control unit and a powersupply unit for supplying a pulse width modulated signal to the at leastone light emitting diode, the control unit being arranged to receive asinput, first and second signals which are representative of theoperating characteristics of the at least one light emitting diode, andwhich is further arranged to control a duty cycle of the pulse widthmodulated signal supplied from the power supply unit to the at least onelight emitting diode in dependence of the first and second signals,wherein the first signal is representative of the current within the atleast one light emitting diode and the second signal is representativeof the temperature of the at least one light emitting diode.
 2. Acontroller according to claim 1, wherein the control unit furtherreceives as input a third signal which is representative of the time anda fourth signal which is representative of ambient lighting conditions.3. A controller according to claim 2, wherein the third signal isgenerated by a clock which is arranged to monitor the time.
 4. Acontroller according to claim 3, wherein the fourth signal is generatedby a sensor which senses ambient lighting conditions.
 5. A controlleraccording to claim 4, wherein the controller is arranged to control theoptical output of a plurality of light emitting diodes.
 6. A controllerclaim 5, wherein the controller is powered by an alternating currentmains supply.
 7. A controller according claim 6, wherein the power unitreceives as input a rectified direct current supply which is obtainedfrom the alternating current mains supply. 8-25. (canceled)
 26. A methodof controlling the optical output of at least one light emitting diode,the method comprising the steps of: providing a pulse width modulatedsignal to the at least one light emitting diode to cause the lightemitting diode to produce an optical output; generating a first signalwhich is representative of the current passing through the at least onelight emitting diode; generating a second signal which is representativeof the temperature of the at least one light emitting diode; adjusting aduty cycle of the pulse width modulated signal supplied to the at leastone light emitting diode in dependence of the first and second signals.27. A lighting system comprising an array of light emitting diodes andat least one controller according to claim 1.